How to do the minimizatipn of the internal parameters using the PORT method

To do that we use the example of the tetragonal structure of TiO2

- Create a new session and its corresponding directory. Generate the structure with the following data (we use a smaller O sphere because Ti-d states are harder to converge then O-p):

Title	TiO2
Spacegroup	(136)
a	8.682 bohr
b	8.682 bohr
c	5.592 bohr
	90
Atom	Ti, enter position (0,0,0) and RMT = 2.0
Atom	O, enter position (0.3,0.3,0) and RMT = 1.6

 - Initialize the calculation using RKmax=6.5 in tio2.in1_st and use 100 k-points and a ``shift`` in kgen. 

-  Start the structure minimization in w2web using ``Execution mini.positions''. This will generate TiO2.inM, and you can try option PORT with tolf=1.0 (instead of 2.0), otherwise stay with the default parameters. Repeat ``Execution mini.positions'' and start the minimization. 

Alternatively you can use 

  min_lapw -p 

 - This will create TiO2.inM automatically, call the program min, which generates  a new struct file using the calculated 

forces, and continues with the next scf cycle.

 It will continue until the forces are below  1 mRy/bohr  (TiO2.inM) and the final results are not ``saved'' 

automatically but can be found in the ``current'' calculation. 

 - You should watch the minimization (:ENE, :FGL002, :POS002) using the file TiO2.scf_mini, which contains the final iteration of each geometry step (see also Sec.5.3.2).

 If the forces in this file oscillate from plus to minus and seem to diverge, or if they change very little, you can edit TiO2.inM (change the method, reduce or increase the stepsize), and remove TiO2.tmpM (contains the ``history'' of the minimization and is used to calculate the velocities of the moving atoms). (This should not be neceaasry for the rutile example, but may occur in more complex minimizations. See comments in Sec. 5.3.2). 

 The final structural parameter of the O-atom should be close to x=0.304, which compares well with the experimental x=0.305.

 

 Our results:

lgerien1970@linux-1h0d:~/WIEN2k/TiO2> grep :POS002 TiO2.scf 
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O
:POS002: ATOM   -2 X,Y,Z = 0.30382 0.30382 0.00000  MULT= 4  ZZ=  8.000  O